Rhodium-catalyzed addition-cyclization of hydrazines with alkynes: Pyrazole synthesis via unexpected C-N bond cleavage

Article abstract of DOI:10.1021/ol501402p

Rhodium-catalyzed addition-cyclization of hydrazines with alkynes has been achieved to afford highly substituted pyrazoles under mild conditions. The cascade reaction involves two transformations: addition of the C-N bond of hydrazines to alkynes via unex

Full text of DOI:10.1021/ol501402p

Letter
pubs.acs.org/OrgLett
Rhodium-Catalyzed Addition−Cyclization of Hydrazines with
Alkynes: Pyrazole Synthesis via Unexpected C−N Bond Cleavage
Deng Yuan Li, Xiao Feng Mao, Hao Jie Chen, Guo Rong Chen, and Pei Nian Liu*
Shanghai Key Laboratory of Functional Materials Chemistry, Key Lab for Advanced Materials and Institute of Fine Chemicals, East
China University of Science and Technology, Meilong Road 130, Shanghai 200237, China
S
* Supporting Information
ABSTRACT: Rhodium-catalyzed addition−cyclization of hydrazines
with alkynes has been achieved to afford highly substituted pyrazoles
under mild conditions. The cascade reaction involves two trans-
formations: addition of the C−N bond of hydrazines to alkynes via
unexpected C−N bond cleavage and intramolecular dehydration
cyclization.
irect addition of carbon−heteroatom bonds to unsaturated
Dcarbon−carbon bonds provides a powerful tool to
construct carbon−carbon and carbon−heteroatom bonds
simultaneously with 100% atom efficiency, fulfilling the require-
ments of green chemistry.1 These transformations usually
require transition-metal catalysts such as nickel,2 palladium,3
platinum,4 rhodium,5 and gold.6 Despite extensive research in
this area, to the best of our knowledge, rhodium-catalyzed
addition of a carbon−nitrogen bond to alkynes has not been
reported.
Substituted pyrazoles are important heterocyclic compounds
used extensively in agrochemical and especially pharmaceutical
applications.7 For example, substituted pyrazoles form the core
of several commercial drugs, including Celebrex,8a,b Acomplia,8c
and Viagra,8d as well as the insecticide Fipronil.8e These
pyrazoles display a broad spectrum of biological activities,
including anti-inflammatory, analgesic, sedative, and hypnotic
properties. They can also act as ligands in coordination
compounds9a and serve as optical brighteners,9b UV stabilizers,9c
and building blocks in supramolecular assemblies.9d
dialkyl ethylenedicarboxylates. As part of our ongoing interest in
constructing heterocyclic compounds, we recently developed
some efficient methods to synthesize useful molecules such as
isochromen-1(1H)-one, isoquinolin-1(2H)-one, and dihydro-
isobenzofuran.18 Here we continue these efforts by using
hydrazine in a rhodium-catalyzed addition−cyclization cascade
reaction with alkynes under mild reaction conditions, producing
highly substituted pyrazoles via formal [3 + 2] cycloaddition
involving unexpected C−N cleavage (Scheme 1).
Initially, N′-phenylacetohydrazide (1a) was treated with
dimethyl but-2-ynedioate (2a) in the presence of [Cp*RhCl2]2
(2.5 mol %) and AgOAc (25 mol %) in MeCN at 60 °C for 5 h.
Interestingly, an unexpected product dimethyl 5-methyl-1-
phenyl-1H-pyrazole-3,4-dicarboxylate (3a) was obtained in
Scheme 1. Rhodium-Catalyzed Reaction of Hydrazines with
Alkynes
The functional versatility of pyrazoles has led organic chemists
to explore different ways to construct them efficiently with
diverse substitutions.10,11 In these approaches, hydrazine has
proven to be one of the most convenient and versatile starting
materials.12 In fact, hydrazines are important synthons for
numerous structurally fascinating heterocyclics, which in turn
can be transformed directly and efficiently into complex
compounds.13 Recently, Glorius,14a Hua,14b and Cheng14c
independently synthesized indole by reacting hydrazine with
alkynes in the presence of a rhodium catalyst. Subsequently,
Kim15 reported a rhodium-catalyzed oxidative olefination of 1,2-
disubstituted arylhydrazines with alkenes to synthesize 2,3-
dihydro-1H-indazoles. More recently, Wang16 achieved the
rhodium-catalyzed synthesis of 1-aminoindole from 2-acetyl-1-
arylhydrazines and diazo compounds.
In 2011, Huang17 reported the synthesis of pyrazoles by the
copper-catalyzed [3 + 2] cycloaddition of phenylhydrazones and
Received: May 16, 2014
Published: June 25, 2014
© 2014 American Chemical Society
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Letter
35% isolated yield (Table 1, entry 1). The structure was fully
consistent with the reported data for 1H and 13C{1H} NMR and
Using these optimized reaction conditions, we probed the
scope and limitation of this cascade transformation (Scheme 2).
Table 1. Optimization of Reaction Conditions for Rhodium-
Scheme 2. Scope of Rhodium-Catalyzed Addition−
a,b
Catalyzed Addition−Cyclization of Hydrazine (1a) with
Cyclization To Form Pyrazoles (3)
a
Alkyne (2a)
b
entry
additive
AgOAc
solvent
temp (°C)
yield (3a, %)
1
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeOH
CH2Cl2
1,2-DCE
THF
60
60
60
60
60
60
60
60
60
60
60
60
60
25
80
60
60
60
60
60
35
ND
71
trace
22
78
52
62
46
61
36
73
88
46
68
10
55
42
0
2
AgSbF6
Cu(OAc)2·H2O
CsOAc
KOAc
3
4
5
6
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
7
8
9
10
11
toluene
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
c
12
d
13
14
15
e
16
f
17
g
18
h
19
i
20
80
a
Reaction conditions: 1a (0.3 mmol), 2a (0.33 mmol), [Cp*RhCl2]2
(0.0075 mmol), NaOAc (0.075 mmol), MeCN (1.5 mL), 60 °C, 5 h,
a
b
c
Reaction conditions: 1 (0.45 mmol), 2 (0.3 mmol), [Cp*RhCl2]2
(0.0075 mmol), NaOAc (0.075 mmol), MeCN (1.5 mL), 60 °C, 5 h,
under N2, unless otherwise noted. Isolated yield. 1a (0.3 mmol) and
d
2a (0.45 mmol) were used. 1a (0.45 mmol) and 2a (0.3 mmol) were
e
f
b
c
under N2, unless otherwise noted. Isolated yield. 1 (0.33 mmol) was
used.
used. [Cp*RhCl2]2 (0.003 mmol) was used. NaOAc (0.03 mmol)
g
h
was used. HOAc (0.3 mmol) was added. Without [Cp*RhCl2]2
catalyst. 1a (7.5 mmol), 2a (5 mmol), [Cp*RhCl2]2 (0.125 mmol),
NaOAc (1.25 mmol), and MeCN (25 mL) were used.
i
First we examined various substituted hydrazines for their ability
to react with dimethyl but-2-ynedioate (2a) to form pyrazoles.
N′-Phenylacetohydrazides with electron-donating Me or MeO at
the para position of the benzene ring gave the corresponding
products 3b and 3c in 86% and 94% yields, respectively. N′-
Phenylacetohydrazides substituted with the electron-withdraw-
ing groups F, Cl, CF3O, or CF3 gave substantially lower yields of
products 3d−3g. These results suggest that the catalytic reaction
prefers electron-donating substituents over electron-withdraw-
ing ones on the benzene ring of N′-phenylacetohydrazide.
Consistent with this idea, substituting the R1 position of N′-
phenylacetohydrazide with meta-Me or meta-Cl and reacting
with 2a furnished the corresponding products 3h and 3i in 81%
and 50% yield, respectively.
The ortho-Me substituted N′-phenylacetohydrazide also gave
the desired product 3j in a slightly lower 60% yield. This may
reflect a steric hindrance that works against cyclization in the
reaction mechanism. Nevertheless, the cascade reaction tolerated
simultaneous Me substitution at both the ortho and para
positions, affording product 3k in 68% yield.
mass spectrometry.11h Inspired by this result, we explored a
variety of reaction conditions in order to optimize the catalytic
process. When AgSbF6 was used as the additive, the product 3a
was not detected, demonstrating the essential role of the acetate
anion (entry 2). The additive Cu(OAc)2·H2O led to the product
3a in 71% yield (entry 3). Finally, NaOAc was found to be the
best choice, giving 3a in 78% yield (entries 4−6). Testing
solvents showed that MeCN led to a higher yield of 3a than did
MeOH, CH2Cl2, 1,2-DCE, THF, and toluene (entries 7−11).
Changing the 1a:2a ratio from 1.0:1.1 to 1.0:1.5 led to a slightly
decreased 73% yield, while inverting the ratio to 1.5:1.0 improved
the yield to a satisfactory 88% (entries 12 and 13). Decreasing the
reaction temperature of 60 to 25 °C or increasing it to 80 °C led
to lower yields of 3a (entries 14 and 15). Similarly, lowering the
loading of catalyst or additive substantially reduced the product
yield (entries 16 and 17). Adding 1.0 equiv of HOAc also
reduced the yield to 42% (entry 18). A controlled experiment
confirmed that the [Cp*RhCl2]2 catalyst was essential for the
formation of pyrazoles (entry 19). Notably, the reaction
proceeded smoothly even on the gram scale, producing 3a (1.1
g) in 80% yield (entry 20).
Next we investigated hydrazines with various substitutions at
the R2 position. Placing a long alkyl chain there led to pyrazoles 3l
and 3m in 76% and 77% yields, whereas a benzyl substituent at
that position gave product 3n in only 50% yield. Adding a
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Letter
substituted aryl to the R2 position gave the desired products 3o−
3q in 91−96% yields. Similarly, naphthyl- or thiophenyl-
substituted hydrazines reacted well, giving products 3r and 3s
in 92% and 82% yields, respectively. Single-crystal X-ray
diffraction analysis of 3q (Figure 1) was consistent with the
addition products 5a and 5b were obtained in 30% yield as
isomers (5a:5b = 2.0:1.0). The structures of both isomers were
verified by single-crystal X-ray diffraction analysis (see
Supporting Information). This result indicates that the amide
N−H bond in substrate 4 undergoes addition with alkyne 2a to
form a C−N bond and suggests that the formal [3 + 2]
cycloaddition proceeds via C−N bond cleavage to afford the
pyrazole.
On the basis of the experimental results and previous studies of
hydrazines that undergo reactions catalyzed by [Cp*Rh-
Cl2]2,14−16 we tentatively propose a mechanism for the cascade
reaction (Scheme 4), although no active intermediate was
Scheme 4. Proposed Mechanism of Rhodium-Catalyzed
Addition−Cyclization of Hydrazines with Alkynes
Figure 1. ORTEP diagram of cascade product 3q.
desired product structure, confirming the predicted structures of
the cascade reaction products. Note that the alkylhydrazines such
as N′-octylacetohydrazide and 1-octyl-2-phenylhydrazine did not
react with 2a to afford the pyrazoles.
To further explore the flexibility of this cascade reaction, we
tried to react N′-phenylacetohydrazide with various substituted
alkynes. Reaction of 1a with diethyl but-2-ynedioate gave
pyrazole 3t in 81% yield. Di-tert-butyl but-2-ynedioate also
reacted with 1a to afford the desired product 3u, albeit in a
slightly lower yield. Interestingly, the reaction of 1,4-
diphenylbut-2-yne-1,4-dione with 1a gave the mixture of
pyrazoles 3v and 3v′ in 56% yield and 23% yield, respectively.
This result revealed that the reaction proceeded via the
intermediate with two ketone groups in the same carbon which
then underwent intramolecular dehydration cyclization to
genearate the products. However, the diaryl or alkyl acetylene
tested did not afford pyrazoles under the optimized reaction
conditions, and the substituted indoles can be detected in the
reactions.14a Note that the unsymmetrical alkynes such as ethyl
but-2-ynoate, ethyl 3-phenyl-propiolate, 4-phenylbut-3-yn-2-
one, (((trifluoromethyl)sulfonyl)ethynyl)benzene, and 1-
((trifluoromethyl)sulfonyl)oct-1-yne could not react with 1a to
give the corresponding pyrazoles.
isolated directly. First, [Cp*RhCl2] reacts with NaOAc to form
the active catalyst [Cp*Rh(OAc)2]. This promotes the
deprotonation of NH in the hydrazine to form the intermediate
A, which subsequently undergoes alkyne coordination to
generate intermediate B. Then intramolecular insertion gen-
erates the six-membered [C−Rh−O] complex C, which
undergoes intramolecular nucleophilic addition to form
intermediate D with a four-membered ring. The following ring
opening cleaves the C−N bond to form the six-membered [N−
Rh−O] complex E. After the protonation, the intermediate F can
be formed and the active catalyst [Cp*Rh(OAc)2] is
regenerated. Finally, the cyclization of F proceeds via intra-
molecular nucleophilic addition and dehydration to afford the
desired pyrazole product.
In summary, we have developed a mild and efficient rhodium-
catalyzed addition−cyclization cascade reaction of hydrazines
with alkynes to afford highly substituted pyrazoles. The formal [3
+ 2] cycloaddition was found to involve the addition of a C−N
bond to alkyne, subsequent cleavage of the C−N bond, and
finally cyclization. This transformation provides a highly concise
and effective protocol to construct the substituted pyrazoles, and
it may expand the usefulness of transition-metal-catalyzed
heterocycle synthesis.
To clarify the cascade reaction mechanism, a controlled
experiment was carried out (Scheme 3). When 4-methoxy-N′-
methyl-N′-phenylbenzohydrazide (4) was reacted with dimethyl
but-2-ynedioate (2a) under optimized reaction conditions, the
Scheme 3. Experimental Investigation of the Reaction
Mechanism
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, characterization data, H and 13C
1
NMR spectra for all compounds, and the X-ray crystallographic
data of 3q, 5a, and 5b. This material is available free of charge via
the Internet at http://pubs.acs.org.
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W. Bioorg. Med. Chem. 2006, 14, 3712. (d) Terrett, N. K.; Bell, A. S.;
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: liupn@ecust.edu.cn.
Notes
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̧ riol,
The authors declare no competing financial interest.
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Commission (Project No. 12ZZ050), the Basic Research
Program of the Shanghai Committee of Sci. & Tech. (Project
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